The present invention relates in general to axial fans, wherein blades are mounted on a hub which rotates to move an air stream from the fan inlet to its outlet along the rotating hub, and in particular to a new and useful method and apparatus for enhancing the static efficiency of axial fans.
In general, fan efficiency is the ratio of work expended to power required. Work can be roughly defined as the product of "Pressure times Airflow". Specifically it is: ##EQU1##
Total pressure is defined as the sum of static pressure and velocity pressure. Static pressure is the suction or positive pressure, normally measured in inches of water, which the fan generates to create the differential pressure needed to move the air. Static pressure is useful work whereas velocity pressure is a parasitic loss and amounts to a loss of energy. The perfect fan would generate all static pressure, i.e., useful work, and experience no velocity pressure loss. One way to minimize velocity pressure is to maximize the net free area between the hub and the fan cylinder or housing. However, increasing the net free area encourages reverse air flow at the hub periphery, near the center of the fan, where the radial velocity of the fan blades is lowest. Thus, we are faced with the dilemma whereby a minimum hub diameter, e.g., 15% to 18% of fan diameter, is desirable to achieve low velocity pressure while a larger hub diameter, e.g., 25% to 30% of fan diameter, is needed to eliminate the negative air currents which occur as a result of low radial velocity of the fan blades near the center of the fan.
As noted earlier, one method of increasing static efficiency is to minimize the hub diameter to fan diameter ratio, i.e., the hub diameter should be small when compared to the fan diameter. Also, in order to perform efficiently, a fan blade must be designed to produce a uniform air velocity profile across the entire outlet area of the fan.
FIGS. 1, 2 and 3 depict the hub and fan blade designs associated with prior art axial fans.
FIG. 1 is a schematic view taken through the radius of a small diameter hub 16 mounted for rotation on an axis 18. A fan blade 14 is attached to the hub 16. A graph which plots fan radius against exit velocity in relative units is superimposed over the hub 16 and the fan blade 14. The fan blade 14 is designed to advance the air in the direction shown at A and to produce a uniform exit air velocity profile, as plotted at curve 12.
An ideal exit air velocity profile has a broad flat high velocity area corresponding to the length of fan blade 14 in the radial direction.
FIG. 2 is a schematic view, similar to FIG. 1, where the same reference numerals are used to designate the same or functionally similar parts as in FIG. 1. However, by contrast to FIG. 1, FIG. 2 depicts a lower efficiency fan blade design 24 which exhibits an exit air velocity profile 22, as determined from the experimental testing of many fans. The fan blade design 24 experiences a large loss of work and efficiency as shown by area 20 between the ideal velocity profile 12 and the actual velocity profile 22.
FIG. 3 is a schematic view, similar to FIG. 1, of still another fan blade design which includes a gap 26 between the hub 16 and the working portion of the fan blade 34. The gap 26 is covered by a seal disc 32 which extends to about 25% to 30% of the fan diameter and blocks some of the negative air vectors. However, the resultant increase in hub area decreases the net free area thereby increasing the velocity pressure which in turn increases the total pressure and reduces static efficiency since: ##EQU2##